The present invention relates generally to cooling gas turbines that are used, for example, for electrical power generation and, more particularly, to the impingement insert to nozzle connection in such gas turbines.
The traditional approach for cooling turbine blades and nozzles is to use high pressure cooling air extracted from a source, such as from the intermediate and last stages of the turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. A combination of external piping and/or internal flow passages are generally used to supply air to the nozzles, with the air typically exiting into the hot gas stream of the turbine to provide air film cooling of the nozzle surface.
In advanced gas turbine designs, improvements in efficiency and reductions in emission can be realized by closed-loop cooling of the hot gas path parts (nozzles, buckets and shrouds). Steam has been demonstrated to be a preferred media for cooling gas turbine nozzles (stator vanes) particularly for combined cycle plants. See for example, U.S. Pat. No. 5,253,976, the disclosure of which is incorporated herein by this reference. However, because steam has a higher heat capacity than the combustion gas, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. Certain areas of the components of the hot gas path, however, cannot practically be cooled with steam in a closed circuit. For example, the relatively thin structure of the trailing edges of the nozzle vanes effectively precludes steam cooling of those edges. Therefore, air cooling may be provided in the trailing edges of nozzle vanes. For a complete description of the steam cooled nozzles with air cooling along the trailing edge, reference is made to U.S. Pat. No. 5,634,766, the disclosure of which is incorporated herein by reference.
In turbine nozzles there are typically impingement inserts disposed inside the nozzle cavities to augment heat transfer coefficients and, therefore, increase cooling of the airfoil walls. The connection of the impingement insert to the nozzle is a difficult task, particularly in closed circuit, steam cooled gas turbine nozzles. While it is desirable for a large physical area to define the connection of the impingement insert to the nozzle, it is not possible to effectively cool such a large area. Therefore, the goal is to minimize the connected area while still maintaining enough strength in the joint. If the joint, which may be for example a braze or weld, should fail, there is no positive retention of the insert. If the joint fails, this would result in a bypass of the cooling circuit, and would cause considerable damage to the nozzle due to the high gas path temperature to which the nozzle is exposed.
Previous designs using air cooling of the airfoil typically retain the inserts using a collar at the end of the insert that attaches to a rib extending radially off the nozzle side wall. Steam cooled (closed circuit) designs may be recessed slightly into the airfoil cavities so that an internal rib, also known as a flash rib, is provided in the cavity for insert attachment. As can be appreciated, recessing the insert into the nozzle airfoil and creating a joint at this internal rib causes difficulty in positive retention in the case of a joint failure. Indeed, no retention radially of the nozzle is provided.
The present invention provides a cooling system for cooling the hot gas components of a nozzle stage of a gas turbine, in which closed circuit steam or air cooling and/or open circuit air cooling systems may be employed. In the closed circuit system, a plurality of nozzle vane segments are provided, each of which comprises one or more nozzle vanes extending between inner and outer walls. The vanes have a plurality of cavities in communication with compartments in the outer and inner walls for flowing cooling media in a closed circuit for cooling the outer and inner walls and the vanes per se. This closed circuit cooling system is substantially similar, structurally, to the steam cooling system described and illustrated in the prior referenced U.S. Pat. No. 5,634,766, with certain exceptions as noted below. Thus, cooling media is provided to a plenum in the outer wall of the segment for distribution therein and passage through impingement openings in a plate for impingement cooling of the outer wall surface of the segment. The spent impingement cooling media flows into leading edge and aft cavities extending radially through the vane. Return intermediate cooling. cavities extend radially and lie between the leading edge and aft cavities. A separate trailing edge cavity may also be provided.
The cooling media that flows through the leading edge and aft cavities flows into a plenum in the inner wall and through impingement openings in an impingement plate for impingement cooling of the inner wall of the segment. The spent impingement cooling media then flows through the intermediate return cavities for further cooling of the vane.
Impingement cooling is typically provided in the leading and aft cavities of the nozzle vane, as well as in the intermediate, return cavities of the vane. More specifically, impingement inserts are disposed inside the nozzle cavities to augment heat transfer coefficients and, therefore, increase cooling of the airfoil walls. The inserts in the leading and aft cavities comprise sleeves that are connected to integrally cast flanges in the outer wall of the cavities and extend through the cavities spaced from the walls thereof. The inserts have impingement holes in opposition to the walls of the cavity whereby cooling media, e.g. steam, flowing into the inserts flows outwardly through the impingement holes for impingement cooling of the vane walls. Return or exit channels may be provided along the inserts for channeling the spent impingement cooling media. Similarly, inserts in the return, intermediate cavities have impingement openings for flowing impingement cooling medium against the side walls of the vane. These inserts also may have return or exit channels for collecting the spent impingement cooling media and conducting it to the cooling media outlet.
As mentioned above, when the insert is recessed into the nozzle airfoil, particularly in the case of a closed circuit cooled nozzle where the joint is created at an internal rib, there is difficulty in positive retention in the case of a joint failure. Thus, the invention provides for positive retention of the insert in the case of joint failure. This retention is achieved by fail-safe tabs and/or standoffs at or adjacent the end of the insert. Fail-safe tabs may be provided as an integral part of the insert structure, and one or more of those tabs may be bent over after assembly to define a retention tab, and brazed in place if desired. The retention/bent-over tabs may be provided to overlie the radial rib and/or the internal rib. It is preferred, however, that the retention tabs be provided only at the radial rib locations for improved cooling, such as to maintain unobstructed cooling of the internal rib connection. In addition or in the alternative to standoffs provided as an integral part of the insert structure, standoffs may be provided to extend off of the adjacent nozzle sidewall cover or impingement plate.
Typically nozzles do not have metering plates as a part of the impingement insert design. Of the known designs using inserted metering plates, the metering plate is placed on and connected to the top of the insert. As used herein, xe2x80x98top of the insertxe2x80x99 refers to the entrance end or inlet end of the insert with respect to the direction of coolant flow therethrough. Thus, intermediate inserts through which coolant flow flows radially outwardly would have a metering plate, if provided, disposed at a radially inner end thereof.
In a second embodiment of the invention, rather than or in addition to providing local tabs and/or standoffs, on one or both ends of the nozzle, an end metering plate is provided that projects laterally beyond the insert so as to overlie the internal rib. The projecting portion of the metering plate thus defines a retention tab(s) for outboard retention whereas inboard displacement limits may be provided by a local standoff projection from the insert and/or the sidewall impingement plate, if provided, or sidewall cover.
Accordingly, the invention is embodied in an impingement insert sleeve for being disposed in a coolant cavity defined through a stator vane, having a generally open inlet end and first and second diametrically opposed, perforated side walls and having at least one tab defined at at least one longitudinal end of the insert for limiting radial displacement of the insert with respect to the vane. In one embodiment, at least one tab is disposed on and extends in a radial direction from an end edge of a main body of the insert, parallel to a longitudinal axis of the insert, for abutting a component facing thereto to limit radial displacement of the insert. In addition or in the alternative, at least one tab projects in a direction generally transverse to a longitudinal axis of the insert.
The invention may also be embodied in a turbine vane segment comprising inner and outer walls spaced from one another; a vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium; an insert sleeve within one cavity and spaced from interior wall surfaces thereof, the insert sleeve having an inlet end through which cooling medium flows into the insert sleeve; the insert sleeve having a plurality of openings therethrough for flowing the cooling medium through the openings into the space between the sleeve and the interior wall surfaces for impingement against the interior wall surface of the vane; and at least one tab defined at at least one longitudinal end of the insert for limiting radial displacement of the insert with respect to the vane.
In another embodiment of the invention, an impingement insert sleeve is provided for being disposed in a coolant cavity defined through a stator vane, the insert sleeve having an inlet end, first and second diametrically opposed, perforated side walls, a collar mounted to the inlet end, and a metering plate having at least one opening for cooling medium flow defined therethrough, the metering plate being mounted to the inlet end of the insert sleeve and including a portion projecting laterally beyond an outer periphery of the insert and the collar. The insert sleeve may further comprise at least one standoff tab projecting radially from a surface of the metering plate, for abutting engagement with an adjacent structure to limit radial displacement of the insert sleeve.
The invention may also be embodied in a turbine vane segment comprising inner and outer walls spaced from one another; a vane extending between the inner and outer walls and having leading and trailing edges, the vane including a plurality of discrete cavities between the leading and trailing edges and extending lengthwise of the vane for flowing a cooling medium; an insert sleeve within one cavity and spaced from interior wall surfaces thereof, the insert sleeve having an inlet end through which cooling medium flows into the insert sleeve, the insert sleeve having a plurality of openings therethrough for flowing the cooling medium through the openings into the space between the sleeve and the interior wall surfaces for impingement against the interior wall surface of the vane; an internal rib being defined about at least a portion of a periphery of the cavity, the insert being mounted to the internal rib by a braze or weld joint; and a metering plate having at least one opening for cooling medium flow defined therethrough, the metering plate being mounted to the inlet end of the insert sleeve and projecting laterally beyond an outer periphery of the insert so as to at least partially overlie the internal rib, whereby radial displacement of the insert with respect to the vane in the event of joint failure is substantially limited.